This invention relates generally to nuclear reactors and more particularly to a heat removal system for the under vessel area of a nuclear reactor.
One known boiling water nuclear reactor includes a reactor pressure vessel (RPV) positioned in a drywell, or containment, and a passive cooling containment system (PCCS). The RPV contains a core, and the containment is designed to withstand pressure generated by the RPV and the core during operation. The PCCS is configured to limit the pressure within the containment to a pressure below a design pressure of the containment and to keep the RPV core substantially cool.
Typically the floor of the containment vessel rests on the reactor building basemat. The basemat may rest on bedrock and typically supports the reactor building and the internal components of the reactor. In the event of a severe accident in which the molten core is postulated to penetrate the lower head of the reactor, the molten core would flow into the under reactor pressure vessel area and begin to attack the floor of the containment vessel and eventually breach the containment boundary.
There are several known methods of protecting the containment floor from the attack of the molten core debris. However, these methods do not include cooling the molten core debris and only prolong the length of time before the molten core debris breaches the containment boundary.
It would be desirable to provide protection for the containment of a nuclear reactor from attack by molten core debris in the unlikely event of a severe accident where the molten core penetrates the lower head of the reactor vessel. It is further desirable to provide protection for the containment that includes cooling the molten core debris to prevent a breach of the containment boundary.
A heat removal system for the under reactor pressure vessel area of a boiling water nuclear reactor provides both protection of the containment boundary from attack by molten core debris and cools the molten core debris to prevent a breach of the containment boundary in the unlikely event of a severe accident where the molten core penetrates the lower head of the reactor vessel. In an exemplary embodiment, the heat removal system includes a glass matrix slab positioned adjacent the floor of the containment and a plurality of heat tubes at least partially embedded in the glass matrix slab and extending into the area under the nuclear reactor pressure vessel. The cooling system also includes fused vent pipes connecting the suppression pool, located in the wetwell, with the drywell, and a passive containment cooling system.
Each heat tube includes an evaporator portion and condenser portion. Each evaporator portion includes a cylindrical evaporator tube and each condenser portion includes a cylindrical condenser tube. Header pipes connect a plurality of evaporator tubes to a plurality of condenser tubes so that the evaporator tubes are in flow communication with the condenser tubes.
At least a portion of each evaporator tube is positioned parallel to the containment floor and embedded in the glass matrix slab. One end of each evaporator tube extends through the glass matrix slab into the drywell area below the reactor vessel and couples with a header pipe.
In the unlikely event of a severe accident where the molten core penetrates the lower head of the reactor vessel, the molten core debris will fall onto the glass matrix slab. The glass matrix slab softens and the uranium and fission products mix with the glass matrix. Because of increased heat in the drywell, the fusible valves open to permit water to flow through the vent pipes from the suppression pool to partially fill the drywell and overlie the condenser tubes. The evaporator tubes embedded in the glass matrix transfer heat from the glass matrix slab to the condenser tubes which then release the heat to the water overlying the condenser tubes. The PCCS removes heat from the interior of the containment and releases the heat to the environment outside the reactor building. As the containment temperature decreases, the glass matrix slab re-solidifies, thus maintaining the integrity of the containment boundary.
The glass matrix slab of the above described heat removal system provides protection for the containment of a nuclear reactor from attack by molten core debris in the unlikely event of a severe accident where the molten core penetrates the lower head of the reactor vessel. Also, the heat tubes and PCCS further provide protection for the containment by cooling the molten core debris to prevent a breach of the containment boundary.